That’s an example of what I hear Spanish psychiatrists call “exogenous depression”. The specific external cause of the depression got solved and the mental state hadn’t become ingrained, so you got out. There was never anything wrong with your chemistry or with your brain processes, what had sucked was your circumstances.
The same happened to my Dad when he got a job after being fired at 50. For any other good news he got between being fired and getting the job, the reaction varied between a very short-lived improvement in mood and being able to go through the polite motions but with no real feeling behind the “that is good news” acknowledgement. Other people will shoot from the depths of despair to the mountains of joy (one of my brothers); problem is, neither state is healthy and also you never know what will kick him down again. Eliahna’s mother sounds like my grandma, but in grandma’s case the bad side is paranoid (in the layman’s sense of the term).